Fundamental knowledge of the way fungi explore the pore volume within a soil is crucial if we are to understand how soil physical conditions affect population dynamics and invasion of many important fungal parasites and saprophytes within soil. In this study, spread of the fungus Rhizoctonia solani Kühn (AG4), an economically important soil-borne plant pathogen and saprophyte, was quantified in relation to tortuosity and continuity of the air-filled pore space. Samples containing fine or coarse sand were equilibrated at matric potentials ranging from −1 to −7 kPa and inoculated with R. solani. We quantified the colony extent and biomass of R. solani spreading from a localized source of inoculum using microscopy and a monoclonal antibody-based immunosorbent assay. Air permeability rapidly increased when the air-filled pore space became continuous below a threshold matric potential of −2.0 kPa for the fine sand and −1.0 kPa for the coarse sand. Fungal spread was limited at high matric potentials with a colony extent of 6.6 and 4.6 mm for the fine and coarse sand, respectively, but increased to 32.6 and 28.5 mm, with a sharp transition below a threshold matric potential of −2.2 and −1.3 kPa. The average colony biomass dropped markedly at the same threshold matric potential. We conclude that the ability of the fungus to invade soil depends on the connectivity and tortuosity of the air-filled pore volume. At near-saturated conditions, the spread was spatially constrained to a relatively small volume, forming small dense colonies. In a well connected air-filled pore volume, larger colonies with low biomass density were formed. The broader implications for invasion of soil by a fungus and for transmission of fungal diseases are discussed.